This invention relates generally to improvements in articulating joint prostheses, particularly such as an improved prosthetic hip joint or the like. More specifically, this invention relates to a combination of an improved ceramic material articulating against a well known and established metal for use in a metal-ceramic composite articulation which exhibits long wear characteristics with substantial elimination of wear debris, and which further exhibits reduced in-vivo fracture risk. Additionally, this invention is also related to the ability to use metal femoral heads with ceramic acetabular liners. The specific clinical benefits of this feature stem from the ability to use fracture resistant heads with low wear and substantial wear debris elimination, the use of large head diameters, which greatly facilitates minimizing risk of dislocation of the head from the prosthetic joint, and providing surgeons and patients with the choice of using this combination for revision of failed joint prostheses.
Typical articulating joints, which consist of a metal surface articulating with a ultra-high molecular weight polyethylene (PE) are inadequate. Clinical studies have shown that the principal cause of implant failure is osteolysis secondary to wear of the implant bearing-surfaces. The primary cause appears to be particulate debris in the form of ultra-high molecular weight polyethylene (PE) released, for example, from the PE acetabular liner bearing of a hip prosthesis1. Such PE wear debris when released into the peri-implant tissues appears to elicit a deleterious biologic reaction, incorporating foreign-body giant cell and macrophage cell responses leading to bone resorption, and eventual loosening of the prosthetic implant. As a consequence, alternative rigid-on-rigid bearing materials such as ceramic-on-ceramic (C—C) (such as alumina), metal-on-metal (M-M), and the recent cobalt chrome alloy (CoCr)—heavily cross linked PE (XPE) are being introduced.
Clinical experience from 1983 to the present has encompassed over two million alumina ceramic femoral-head implants.2, 3 Total hip replacement studies incorporating both CoCr and alumina ceramic heads have established the superiority of ceramic-PE couples over metal-PE couples, with alumina-alumina couples demonstrating 2-3 orders lower wear volume than the best ceramic-PE couples.4 Even so, the major limitation to use of alumina ceramics today is the likelihood of brittle fracture, even in just a low incidence of 2% or less. From the limited series of clinical studies available in the United States, the failure incidence of alumina heads was found to be surprisingly high and of quite short follow-up periods, anywhere from 9 months to 10 years.6, 5 Thus the fracture incidence in ceramics is still of clinical concern. Typical ceramic materials have low toughness and are prone to failure by brittle fracture. As history has indicated, there is an urgent need to find an improvement to alumina, particularly with ceramic-ceramic couples which have higher bearing contact stresses.6 
Low wear of articulating components occurs when the mating surfaces have comparable and high hardness, good surface finish, conformal surface geometry, compatible mechanical properties and a low coefficient of friction. It is because of the first three conditions that ceramic-ceramic couples have demonstrated very low wear. Contact damage results in the weaker material when the moduli and hardness of the articulating surfaces are very different, as is the case for CoCr-PE or even zirconia or alumina ceramic-PE. An ideal articulating low wear couple will have closely matching properties and high toughness. Traditional ceramics such as alumina are prone to brittle fracture owing to their low toughness. Such brittle failure in ceramic materials results from propagation of microcracks initiated at and just below the surface. Other ceramic materials such as zirconia, zirconia toughened alumina or Si3N4 that have higher toughness have significantly higher reliability than alumina, owing to the ability to avoid catastrophic failure. Specifically, using such ceramics can allow significant improvements in wear properties along with improved reliability. The specific advantages can be illustrated by considering the articulating hip joint. If the articulating hip joint can be made with a metal femoral head and a ceramic acetabular cup, additional significant clinical benefits can be obtained as listed below:                The metal femoral head does not fail catastrophically as ceramic heads can, assuring patient safety;        The metal head can be made of a larger size, up to 44 mm diameter, than ceramic heads can typically be made, providing the surgeon greater flexibility in implant size selection; and        The metal head can be used as both a primary hip prosthesis or a revision hip prosthesis.        
This invention describes a Si3N4 acetabular cup-CoCr femoral head couple. This couple is superior to other ceramic-metal couples such as alumina-metal,7,8,9 owing to compatible properties such as hardness, tensile strength, elastic modulus, high fracture toughness, and lubricity. Si3N4 also has an optimal combination of toughness and strength properties that gives superior damage resistance: the ability to retain strength following contact damage. Friction property studies of Si3N4 ceramics show that Si3N4-(M-50) steel hybrid bearings and Si3N4—Si3N4 bearings had the lowest friction coefficients under both lubricated and dry conditions of the materials tested. In contrast, alumina ceramic-ceramic and alumina-steel bearings had approximately three times the friction coefficient when tested under similar conditions.
It is therefore an object of this invention to provide a new set of bio-compatible articulating surface materials for use in prosthetic joints which will have:                Ultra-low wear with volumetric wear rates of less than 1 mm3/10 million cycles;        Long in-vivo life;        Wide range of sizes maximizing surgeon choice and optimizing fit to patient anatomy;        Wide bio-mechanical margin of safety for all sizes, minimizing risk of in-vivo fracture;        Preserving modularity of prosthetic articulating joint designs; and        Allowing both primary and revision prosthetic articulating joint designs.        